This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Coronaviruses, a significant health threat to humans and other animals, have perplexing receptor recognition patterns. Human NL63 respiratory coronavirus (NL63-CoV) is the only group-I coronavirus known to use angiotensin-converting enzyme 2 (ACE2) as its receptor. The other group-I coronaviruses use aminopeptidase-N (APN) from their respective hosts. Curiously, ACE2 is also used by group-II human SARS coronavirus (SARS-CoV). Another group-II coronavirus, mouse hepatitis virus (MHV), uses a cell adhesion molecule CEACAM1a as its receptor. Defined receptor-binding domains (RBDs) on the spike proteins of coronaviruses are responsible for high-affinity binding to their receptors. MHV is the only coronavirus that uses the N-terminal domain of its spike protein as its RBD, whereas all other coronaviruses with known receptors use other domains of their spike proteins as their RBDs. This research investigates how coronaviruses recognize their receptors and how they interact with receptors from multiple hosts. Specifically, we will determine crystal structures of viral RBDs complexed with their receptor, use surface plasmon resonance to examine interactions between viral RBDs and their receptors, and use pseudotyped viral infection and cell-cell fusion assays to examine interactions between viral spike proteins and their receptors. This research has significant scientific and medical implications. It will not only explore novel principles governing viral evolution, virus-receptor interactions, viral host ranges and cross-species infections, but also guide antiviral efforts.